This invention relates to electronic cameras and, more particularly, it concerns a method and apparatus for exposure control in electronic cameras having video and still modes of operation.
NTSC video cameras are well known in the art and conventionally employ an optical system to present light images to a solid state sensor, such as a charged couple device (CCD), at effective electronic shutter speeds of 1/60 second to generate 30 image frames per second using 488 interlaced lines at the 15.7 kH.sub.z rate. A timing diagram depicting the operation of a conventional NTSC system in both video (motion picture) and still modes of operation appears in FIG. 1 of the attached drawings. In the video mode of operation, the length of time during which each of a Field A and a Field B is integrated or exposed to image light is controlled electronically without a mechanical shutter mechanism by alternately integrating and discharging or reading out the respective two fields in synchronism with timing control supplied by a CCD Clock. Such electronic shutter operation is due to the characteristics of a CCD, namely, it becomes relatively insensitive to light during readout. Moreover, the same electronic shutter operation may be used when the camera is switched to a still mode of operation by clearing the CCD (again in synchronism with CCD Clock timing), integrating the CCD to the desired still image and terminating CCD exposure by readout. In this connection, it is to be noted that the still mode of operation employs a different method of CCD signal formation and readout than the video mode but the exposure sequence for both is as depicted in FIG. 1 of the attached drawings.
From the foregoing, it will be seen that a conventional NTSC video camera can be operated using the electronic shutter feature in both video and still modes of CCD exposure; subject, however, to the use of the same 1/60 second shutter speeds in both modes of operation. While shutter speeds of this order are acceptable for motion picture images, it is apparent that a high quality still photograph of a moving subject will require shorter exposure times.
The problems associated with the shutter speed requirements of video/still or dual mode video cameras have been addressed in the prior art. For example, U.S. Pat. No. 4,551,764 issued Nov. 5, 1985 to Kimiaki Ogawa discloses a video camera system in which an iris diaphragm, normally adjusted automatically as a variable lens aperture for operation in the video mode of operation, is converted to a mechanical shutter for CCD exposure in a still mode of operation. While the disclosure of the Ogawa patent appears to achieve shutter speeds faster than 1/60 second, no specific still mode shutter speeds are mentioned. It is generally known, however, that shutter speeds as fast as 1/250 or 1/500 second are difficult, if not impossible, to achieve using mechanisms of the type represented by iris diaphragms used for automatic aperture control. In addition, no provision is made in the disclosure of the aforementioned patent for aperture adjustment in the still mode of operation. While the extent to which the iris diaphragm is open may provide for aperture adjustment, such operation means that as shutter speeds increase the blades of the iris diaphragm must move further to provide a corresponding increase in aperture size. In other words, faster effective shutter speeds would require more mechanical shutter component movement to achieve the needed larger aperture stop for proper exposure than slower shutter speeds which require smaller apertures. In addition, the mechanical components of the iris diaphragm must be switched from one in which it is controlled automatically by the exposure control system operative in the video mode to one in which it must be abruptly closed to terminate a still exposure of the CCD. As a result, the total duration of time required for exposure of the CCD to generate a still photograph and then return to normal motion picture video operation is increased.
In light of the foregoing, there is a need for an improved exposure control system for video cameras of the type referred to.